Mixtures of oxides of nitrogen, commonly called NO.sub.x gases are generated as by-products in combustion processes such as in automotive engines or in fossil fuel power plants. These oxides are hazardous to health and the environment as they produce acid rain.
Up to the present time NO.sub.x emissions in automotive and stationary power plants have been controlled by reducing them to nitrogen by a three way catalyst (TWC) such as Pt and/or Pd+Rh!/CeO.sub.2 --Al.sub.2 O.sub.3 and selective catalytic reduction (SCR) using a catalyst such as V.sub.2 O.sub.5 /TiO.sub.2 or Fe,Cu,etc!-Zeolite respectively.
In a low oxygen content system, rhodium (Rh) selectively reduces NO.sub.x to N.sub.2 in stoichiometric ratio, that is, in a ratio of reducing agents to oxidizing agents of about 1, whereas Pt catalyzes the reduction of NO.sub.x to ammonia. This is an undesirable product so that even though platinum is used in automotive catalysts, enough Rh must be present to catalyze reduction to N.sub.2. In SCR reactions an external ammonia source is needed to reduce NO.sub.x to N.sub.2 over vanadia or zeolite catalysts. Thus it has been impossible to use SCR catalysts and reactions in automotive exhaust cleaning up to now because supplying NH.sub.3 in precise amounts is difficult and impractical.
U.S. Pat. No. 3,767,764 relates to a method for converting hydrocarbons, carbon monoxide, and nitrogen oxides in auto exhaust gases to innocuous entities which comprises contacting the gases from half of the engine cylinders with a catalyst specific for reducing nitrogen oxides to ammonia in a first catalytic converter, mixing the portion with air to provide a net oxidizing atmosphere and with the balance of the exhaust gases and contacting the mixture with an oxidation catalyst in a second catalytic converter to convert the hydrocarbons, carbon monoxide, and nitrogen oxides to innocuous entities.
It would be highly desirable, therefore, to have a catalyst system which would be able to catalyze reactions of combustion exhaust gases efficiently and economically without rhodium and without having to supply external ammonia or oxygen thereto.